In a large pan, heat the oil, sugar and pepper over a low heat. Add the onions & garlic and stir so they're coated in oil/sugar. Place the sprigs of thyme in - if you have kitchen string and can tie them together, makes them easier to remove later. Put lid on pan, stir two or three times over the next 15 minutes.
When the onions are very soft, remove the thyme and add the beef stock and increase the heat til it's simmering then reduce back to low. Add the mustard and stir it in well. Leave to simmer for another 10 minutes.
As you'll want 4 bowl-shaped pieces for the top of the soup, cuts each slice into a suitable 'round' and place any off-cuts in the bottom of the bowl to soak up the soup, as they're just there to absorb liquid and provide a filling meal.
Fill the bowls nearly to the top with soup and then place the bread rounds on top each soup. Press down slightly so they absorb the liquid. Then top with the cheese and place under a hot grill until the cheese is bubbling.